It is known in the field of signal processing that frequencies close to DC (i.e. zero frequency) are difficult to modulate. One prior art circuit used to overcome this problem is a dual port frequency modulator. The dual port frequency modulator is designed to modulate frequencies from zero frequency and greater.
The conventional dual port frequency modulator includes two data processing paths: a high frequency processing path for modulating high frequency components of the modulation signal and a low frequency processing path for modulating low frequency components. The high frequency processing path typically comprises a buffer and a variable gain amplifier which function to account for frequency deviation and gain imbalances, respectively, between the high and low processing paths. The low frequency processing path, on the other hand, comprises considerably more signal components than the high path due to the additional signal processing needed to modulate lower frequency signals. The lower path often includes a series coupled temperature controlled crystal oscillator (TCXO), two frequency counters, a first phase detector, and a loop filter. The outputs of the high and low frequency paths are both coupled to a separate input of a voltage controlled oscillator (VCO). The VCO effectively adds the two input signals and outputs the desired frequency modulated signal.
The main drawback for the above described modulator design is that, due to the different structures of the high and low paths, the systems have different time delays between the modulation input and the inputs to the VCO. This time delay difference equates to a phase delay difference between the high and low paths which results in a non-linear phase response in the vicinity of the loop filter cut-off frequency for the above described dual port modulator. Due to this non-linear phase response, signals in the vicinity of the loop filter cutoff frequency have approximately the same amplitudes but different phases. As a result, out-of-phase signals cancel and some attenuation occurs in the output signal of the modulator, thereby affecting the impulse response (i.e. gain and magnitude response) of the modulator. The manner in which the impulse response is affected is that the gain of the modulator in the region of the loop filter frequency "dips". The magnitude response, in turn, affects the modulation index, and in particular, degrades the frequency deviation versus time response of the prior art modulator.